The present invention relates to a multilayer-coated substrate for use as an information recording medium utilizing magnetism or light or as an optical part for optical communication, etc., and to a process for producing the same. More particularly, the invention relates to a multilayer-coated substrate for use as an information recording medium, such as a CD-ROM, or as an optical part, such as planar microlenses or a grating element, and a process for producing the same.
Information recording media such as CD-ROMs and optical parts such as planar microlenses or grating elements should have minute projections on the surface thereof. These minute surface projections function as pits or tracking guides in the information recording media. In the optical parts, the surface projections converge or diffuse light to function as microlenses or a diffraction grating.
A technique for forming these surface projections is known which comprises evenly spreading an ultraviolet-curable resin on a substrate and irradiating the resin with ultraviolet while pressing the resin with a die having recesses (Unexamined Published Japanese Patent Application No. 63-49702).
In Unexamined Published Japanese Patent Applications Nos. 62-102445 and 6-242303 is described a process for production by the so-called sol-gel method, which comprises applying a solution containing a silicon alkoxide on a glass substrate and heating the applied solution while pressing a die having recesses against the same to thereby form projections.
Furthermore, Unexamined Published Japanese Patent Application No. 63-197382 describes a technique in which an ultraviolet-curable resin is evenly spread on a substrate and projections are formed thereon by reactive ion beam etching using as a mask a pattern formed with a photoresist.
However, the conventional techniques described above have had the following problems. First, the ultraviolet-cured resin has low heat resistance and decomposes or yellows when heated to 250xc2x0 C. or higher. Consequently, the substrates having projections made of the ultraviolet-cured resin are incapable of heat processing such as soldering and it has been difficult to attach the substrates to apparatus, etc.
In contrast, the silicon alkoxide projections formed by the sol-gel method have high heat resistance and are capable of soldering, etc. However, the sol-gel method has had a problem that a thick film cannot be formed. When a silicon alkoxide layer of tens of micrometers is actually formed by the sol-gel method, the surface thereof develops minute cracks (hereinafter referred to as cracks). This is because as the silicon alkoxide solution gels and solidifies, the layer comes to have a difference in the progress of condensation polymerization reaction between the surface and an inner part thereof and, hence, a high stress generates on the surface. There have even been cases where the layer peels off the substrate due to the stress.
The technique of forming projections by reactive ion beam etching has had problems that the production steps are complicated, resulting in an increased production cost, and that it is difficult to improve evenness of the projections. Still another problem is that because of these, it is difficult to form a layer having a large surface area.
The invention has been achieved in view of the problems encountered in such conventional techniques. An object thereof is to provide multilayer-coated substrates in which the coating has high heat resistance, neither develops cracks in its surface nor peels off the substrate even when thick, and has a layer having uniform projections even when having a large area. Another object is to provide processes for producing these multilayer-coated substrates easily at low cost.
These objects have been accomplished with the following multilayer-coated substrates of the invention and the following processes for producing the same.
1. A multilayer-coated substrate comprising a substrate and united therewith two or more superposed layers which comprise an organopolysiloxane and the outermost layer of which has projections, the projections having a dispersion of height of 1 xcexcm or less.
2. The multilayer-coated substrate described in item 1 above wherein the projections of the outermost layer have at least one sectional shape selected from the group consisting of a circular arc, an elliptic arc, and an angle.
3. The multilayer-coated substrate described in item 1 or 2 above wherein a lower layer also has projections conforming to the projections of the outermost layer.
4. The multilayer-coated substrate described in any one of items 1 to 3 above wherein in the two or more layers, the ratio of the thickness of the thickest layer to that of the thinnest layer is from 1 to 5.
5. The multilayer-coated substrate described in any one of items 1 to 4 above wherein in the two or more layers, the coefficients of linear expansion of the respective layers change gradationally from the substrate toward the outermost layer.
6. The multilayer-coated substrate described in any one of items 1 to 5 above wherein the two or more layers are two layers.
7. The multilayer-coated substrate described in any one of items 1 to 6 above wherein the substrate is a transparent body.
8. The multilayer-coated substrate described in item 7 above wherein the two or more layers gradationally change in refractive index from the substrate toward the outermost layer.
9. The multilayer-coated substrate described in item 7 or 8 above wherein the two or more layers satisfy the relationship
tx/nx=xcex/4
wherein tx is the thickness of an arbitrary layer, nx is the refractive index thereof, and xcex is the wavelength of the transmitted light.
10. The multilayer-coated substrate described in item 9 above wherein the two or more layers satisfy the relationship
na/nb=(ns/no)
wherein no is the refractive index of the outermost layer, nb is the refractive index of an intermediate layer, na is the refractive index of the innermost layer, and ns is the refractive index of the substrate.
11. The multilayer-coated substrate described in item 9 or 10 above wherein the transmitted light has a wavelength of from 380 to 2,000 nm.
12. The multilayer-coated substrate described in anyone of items 1 to 10 above wherein in the two or more layers, the outermost layer has been formed from methyltriethoxysilane and a lower layer has been formed from methyltriethoxysilane or tetraethoxysilane.
13. A process for producing a multilayer-coated substrate which comprises pouring a solution of the organopolysiloxane on a substrate and into molds, separately causing the solution layers to gel, subsequently superposing these gels on the substrate, and uniting the superposed gel layers with the substrate while pressing the gels with the mold for the outermost layer.
14. The process for producing a multilayer-coated substrate described in item 13 above wherein the mold for an arbitrary layer is one with which the layer is made to have the shape as described in item 2 or 3 above.
15. The process for producing a multilayer-coated substrate as described in item 13 or 14 above wherein the temperature at which the organopolysiloxane solution is caused to gel is from 20 to 120xc2x0 C. and the temperature at which the superposed gel layers are united with the substrate thereafter is from 50 to 150xc2x0 C.
16. The process for producing a multilayer-coated substrate as described in any one of items 13 to 15 above wherein the gels to be superposed have a viscosity of from 1xc3x97104 to 1xc3x97106 P.